In an attempt to obtain better photovoltaic cells, heterojunction devices have been investigated. Nevertheless many heterojunction devices exhibit various problems including those associated with the interface of two different materials, e.g., lack of adhesion.
The Cu.sub.x S/CdS cell, which has received the greatest amount of development work for terrestrial application, exhibits up to 3.7% lattice constant mismatch (See Tseng and Greenfield, Physica Status Solidi, 26, 643-656 (1974)) which is expected to cause large interface defect densities. Additionally, the Cu.sub.x S/CdS cell has other potential difficulties. For example, copper sulfide is unstable in an atmosphere containing oxygen or water. The multiphase character of copper sulfide presents additional difficulties. Between 450.degree. C and room temperature copper sulfide displays multiple phase changes. This property makes production of defect free crystals more difficult. Indeed it seems possible that heat from solar radiation itself could cause local deformities where the solar light has been focused on the cell.
Other heterojunctions have also been experimented with but no particular heterodiode has shown clear superiority.